Filter beds formed from one or more layers of filter media have been employed in a variety of known filters for filtering water or wastewater to remove impurities from liquids. For example, filter beds of granular media have been used in upflow filters, downflow filters as well as other types of filters including bi-flow filters. After the filter has been operating for a while, it is necessary to wash the filter bed to remove the impurities trapped in the filter bed during the filtration mode. Various methods have been used to wash the filter bed including but not limited to the steps of: (i) liquid only wash; (ii) air only wash; (iii) liquid and air concurrently; (iv) liquid only followed by air only; (v) air only followed by liquid only; and, (vi) liquid and air concurrently followed by liquid only.
It is important that the washing fluid is thoroughly distributed through the filter bed during the washing mode in order to remove an adequate amount of the impurities trapped in the filter bed during operation of the filter in the filtration mode. It is similarly important to uniformly distribute in-service fluids during the filtration mode. Various underdrains systems have been used in an attempt to distribute fluids uniformly throughout the filter bed.
A common underdrain system includes a distribution chamber and a plurality of branches extending from the distribution chamber. In these types of systems, fluids (e.g., washing fluids and/or in-service fluids) are introduced into the distribution chamber (e.g., flume, gullet, plenum, header, etc.). As used herein “washing fluids” includes but is not limited to fluids directed in a direction opposite to the direction of filtration as well as fluids directed in the same direction as the direction of filtration. The fluid is then directed through a plurality of branches (e.g., laterals) operably connected to the distribution chamber. The laterals include openings that allow the fluid to be released into the filter bed. The distribution chamber and branches can take many different forms.
There have been many efforts directed at uniformly distributing fluids through the branches extending from the distribution chamber. Mal-distribution of fluids in this type of underdrain system stems from the fact that the initial velocity of the fluid in the distribution chamber is high and decreases over the length of the distribution chamber as fluid is diverted to various underdrain laterals or the like. This change in the velocity of the fluid over the length of the distribution chamber changes the velocity head over the length of the distribution chamber. Velocity head is an expression of how much energy is tied up in the momentum of the fluid. Bernoulli's equation is a version of the fundamental energy equation streamlined to describe fluid flow. Per this equation, an increase in velocity head will reduce the amount of piezometric or pressure head available. As a result, the branches near the fluid inlet in the distribution chamber, where the fluid velocity is the highest, will be exposed to a lower piezometric pressure. This results in a reduced flow of fluids through the branches located near the fluid inlet as compared to branches disposed further from the fluid inlet. Accordingly, the flow of fluid through the branches is not uniform. Hence, the fluid is not uniformly distributed to the filter bed that is undesirable in either a filtration mode or a washing mode.
A variety of methods and systems have been used to counteract the above-described mal-distribution of fluids through a filter bed. One attempt to overcome the mal-distribution of fluids requires reducing the velocity of the washing fluid introduced into a flume (e.g., center flume, end flume, etc.) to a maximum washing velocity of approximately 2 to 4 ft/sec. Other methods include tapering the flume cross-section in an effort to reduce changes in flume velocity, employing baffles to change the entrance flow characteristics of the washing fluids and using individual orifice plates at individual lateral entrances to match the lateral entrance losses to the available pressure head. These methods vary as to their degree of success in overcoming the mal-distribution of fluids. Also, these methods are often cost prohibitive to implement. Further, these methods are difficult or impossible to retrofit into an existing filter.